Recently, tunable or reconfigurable microwave devices have no doubt drawn much attention due to their increasing importance in improving the performances of the current and future wireless communication systems. In response to this requirement, various kinds of frequency-tuning techniques, such as RF MEMS, semiconductor diode, ferroelectric material and so on, have been developed and applied in the designs of microwave tunable circuits and components. Among them, varactor diode is widely used to tune the operation frequency due to its high tuning speed and reliability.
As well known, tunable transmission line resonator has played an essential and key role in the development of tunable microwave components and circuits. Being widely used in many practical designs, tunable one guided-wavelength (λg) ring resonator is one of the notable examples. Besides study and application of itself, derived from which, a λg/2 resonator of open-ended or short-ended is also widely studied and applied, and become a key component of the designs of microwave circuits. Nevertheless, as can be seen from the previous publications, no matter where the loading capacitors are placed along or no matter how many capacitors are attached to the ring resonator, the tuning range of the fundamental resonant frequency (ffund) is always f0→f0 where f0 is the fundamental resonant frequency of the initial-state ring resonator. The operation principle is that ffund is generally shifted down as the loading capacitances are increased. Obviously, the limited tuning bandwidth of ffund will become a problematic issue in the tunable and reconfigurable wireless systems, which needs to be addressed.